Bulletin of Entomological Research (1997) 87, 101-104 101 SHORT COMMUNICATION Metathoracic sensilla in mosquitoes (Culicidae) and related Diptera, and their potential importance in taxonomy and behavioural physiology Ralph E. Harbach and Theresa M. Howard Department of Entomology, The Natural History Museum, London, UK Introduction The sensory structures of mosquitoes were thoroughly reviewed by Mclver (1982). Numerous sensilla which function as contact and olfactory chemoreceptors occur on the antennae, maxillary palpi, mouthparts, and tarsi of adult mosquitoes. In addition, the bodies of mosquitoes bear a variety of setae which light microscopic studies have shown to be innervated in a manner indicative of a mechanosensory function. Mosquitoes are also extensively clothed with scales. Scales are flattened setae with longitudinal ridges (Harbach & Knight, 1980). They arise from alveoli (sockets) like ordinary setae, but are not innervated. The presence or absence of setae and scales on the thorax of mosquitoes are of considerable taxonomic importance. While investigating the occurrence of setae and scales on the metathorax of certain sabethine mosquitoes for the development of improved keys to the mosquito genera of the world, we discovered four small setae (sensilla) located below the base of the halter and posterior to the metathoracic spiracle. This discovery prompted us to look for these setae in other mosquitoes and related Diptera. Our findings are the subject of this communication. Materials and methods This study is based on the examination of dried museum specimens. The metathoracic sensilla were initially discov- ered and subsequently studied with scanning electron microscopy. Specimens were removed from pins, glued to stubs with araldite *, and sputter-coated with palladium. The species studied are listed in table 1. These include 42 species of Culicidae from 29 genera (38 genera are currently recognized) and 13 species of other Diptera representing eight families, mainly Nematocera. Observations were made on both a male and a female of 19 mosquito species and either a male or a female of the other species. The sensilla Correspondence: Ralph E. Harbach, Department of Entomology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK. were examined on one side of the thorax only. Specimens and species were chosen primarily on the basis of availability. Results and discussion All of the species examined in this study have a group of similar sensilla located just below the base of the halter and posterior to the metathoracic spiracle (fig. 1A,B). Phyloge- netic homology of these sensilla in different families is presumed on the basis of position and appearance. The number of sensilla observed in the various specimens and species are indicated in table 1. Four sensilla were present in all mosquitoes except for three specimens: a female of Armigeres malayi (Theobald) had three sensilla, and a female of Tripteroides dolfeini (Giinther) and a male of Mansonia africana (Theobald) each had five sensilla. Since four sensilla were observed in the opposite sex of the first two species, the presence of three and five sensilla appears to be an anomaly. The opposite sex of the latter species was not examined. The number of setae present in non-culicids ranged from two in Culicoides nigeriae Ingram & Macfie (Ceratopogonidae) and Dixa fusca Loew (Dixidae) to seven in Epiphragma ocellare (Linnaeus) (Tipulidae). Too few species from families other than Culicidae and Simuliidae were observed to determine whether the number of setae are constant across generic lines (as they are in mosquitoes) or variable in number between genera and subgenera within families (as they are in blackflies). The sensilla typically arise from rather large, rimmed alveoli and are often directed toward the spiracle (anteriad) (figs 1A,B; 2B-D). The alveoli are usually simple depressions (figs ID, 2D), but in several sabethine mosquitoes the inner wall is fluted (fig. 2A). The four sensilla of mosquitoes are arrayed in a more or less quadrangular pattern (fig. 1B,D), with the exception of the two Anopheles species where they are more widely separated in a vertical direction (fig. 1C). In those species with three sensilla, Chaoborus edulis Edwards (Chaoboridae) and Corethrella calothicola Edwards (Corethrel- lidae), the sensilla are inserted in a straight vertical row (fig. 2C). In comparison, the atypical female of Armigeres malayi